Magnetic compasses especially adapted for use in automotive vehicles are commonly used as an aid in orientation for the purpose of highway navigation. Typically, such compasses are of the type which utilize a compass card which carries the compass magnet as well as the direction indicator within the view of the vehicle driver. Compensation of the compass for magnetic field deviation due to the local effect of magnetic materials and for magnetic variation due to geographical location has long been a problem in obtaining reliable direction indication with such compasses. Improved compensation and also improved direction display for the driver can be achieved by locating the magnetic field sensor of the compass and the compass display at separate locations in the vehicle. For this purpose, it is desirable to utilize an angular position encoder with the compass for producing electronic direction data signals and applying such signals to a remote display for presentation of the direction information to the driver. Such an automotive vehicle compass is disclosed in the Dinsmore patent 4,402,142 granted September 6, 1983. The compass of this patent is provided with a Hall effect angular position encoder for producing electronic direction data signals.
Magnetic compasses are well known which utilize optical devices for angular position encoders for producing electronic direction data signals; however, such compasses known in the prior art are not well adapted for use in automotive vehicles because of complexity, size and cost. A compass for automotive vehicles must be adapted for mass production at low cost and yet it must provide an accuracy consistent with the need for navigation of an automotive vehicle. The compass must be of rugged construction and reliable in operation. It is desirable to have the compass magnetic field sensor and the angular position encoder adapted for mounting in a variety of locations in the vehicle so as to facilitate space utilization and to minimize the adverse effects of magnetic field deviation. Preferably, it should be adapted to produce electronic direction data signals for application to a remote display. The compass, i.e. the combined magnetic field sensor and angular position encoder, should be of small size. It may be desired to mount the compass on a circuit board containing the electronic circuit required for processing the data signals and for driving the remote display device to show the heading of the vehicle. The Schulte patent 2,518,258 granted August 8 , 1950 discloses a magnetic compass with photoelectric means for indicating deviations from a set course. In this compass, a semi-spherical compass card is disposed in a housing and supported by a pivot pin on a central post. The post supports a light source and a pair of photocells are mounted on the compass housing at diametrically opposite locations. The compass card is provided with a window of such angular extent that it blocks light from the light source from impinging upon either of the photocells when the heading coincides with the set course. However, when the heading deviates from the set course the window allows the light to impinge upon one photocell or the other, depending upon the direction of deviation.
The Fowler patent 3,746,842 granted July 17, 1973 discloses a magnetic compass with provides a digital signal representing an indication of direction. A rotor, in the form of a circular disk, is supported in a frame on a pivot pin which also carries a compass bar magnet. The circular card bears a coded mask and a light source on one side of the mask and a photoelectric detector on the other side generate a digitally coded signal corresponding to the position of the rotor with respect to the frame. The coded mask on the card comprises a plurality of annular tracks of different radius. The tracks constitute a code representing angular position with each track comprising alternate light transparent and opaque sectors. The tracks are coded in a binary code such as the Gray code. In this each individual track contributes one bit of a binary number and hence the degree of resolution of angular position is determined by the number of tracks. A separate photoelectric detector is provided for each track and the set of detectors generate a digital electronic signal corresponding to angular position. Means are provided for converting the electronic signal to a decimal course display. Remote reading magnetic compasses utilizing similar optical position encoders are described in the Fowler patent 4,047,168 granted September 6, 1977 and the Lapeyre patent 4,414,754 granted November 15, 1983.
The Edwards patent 4,146,970 granted April 3, 1979 describes a remote indicating magnetic compass for use in an automotive vehicle. According to this patent, the system comprises a compass which may be mounted at a selected location on the vehicle and a bearing display device which may be mounted on the instrument panel of the vehicle. The compass comprises an encoding wheel which is rotatably mounted within a housing on a pivot pin and which carries a compass magnet. The encoding wheel is provided with one or more code tracks each of which comprises a segment of light reflective surface and a segment of non-reflective surface. Four light source-sensor pairs are spaced circumferentially around the wheel with the light paths between the source and sensor of each pair impinging upon the surface of the code track on the wheel. The light source-sensor pairs provide a digital signal which represents the angular position of the compass wheel. The degree of resolution for determining the angular position is dependent upon the number of code tracks on the wheel.
The Kramer patent 3,950,859 granted Apr. 20, 1976 discloses a magnetic compass with a digitally coded disk for determining angular position. The coded disk is generally transparent but includes a sequence of opaque areas in a circumferential track. There are a plurality of tracks and each track produces a sequence of binary digits which form a bit in a binary coded decimal character. The coded disk is disposed between a pair of plates which are provided with aligned apertures for each of the tracks on the disk. For each track, a light emitting diode is positioned behind one plate and a photo transistor is positioned behind the other plate for generating the digital signal corresponding to that track.
Although magnetic compasses are well known which utilize angular position encoders for producing electronic direction signals for remote display, such compasses as are known in the prior art are not well adapted for use in automotive vehicles and small boats because of complexity, size and cost. A compass for such vehicles must be adapted for mass production at low cost and yet it must provide an accuracy consistent with the need for the vehicle navigation. The compass must be of rugged construction and reliable in operation and it must lend itself to mounting in a variety of locations in the vehicle. The compass, i.e. the combined magnetic field sensor and the angular position encoder, should be of very small size.
A general object of this invention is to provide an improved magnetic compass with an optical position encoder which overcomes certain disadvantages of the prior art.